In Leopard, the newest version of Apple's OS X operating system, the company has mentioned 300 or so new features. But I'm a Mac developer and user, and I've been using it from the inside. So what's really there?

The first thing a Mac user will notice on upgrading to Leopard is the changes - both subtle and dramatic - to the desktop's appearance. Apple puts this at the very top of its list of new Leopard features: the menu bar, dock and Finder windows have all received a makeover. Most of the updates and additions - here's Apple's full list - can be considered "subtle but very useful".

My personal favourite feature in Leopard is the improvements to Spotlight, the file searching system (which I'll deal with later): now that it's much faster, I can see it becoming a regular feature of my daily work, rather than a tool of last resort. I'm also a huge fan of Stacks, which lets you view the contents of a folder rapidly from the Dock.

Another small enhancement I find delightful - being an internet vagabond, hopping from one wireless network to another - is that Leopard's Wi-Fi network menu now shows you which networks in your range are secured and which are open.

That's the thing about Leopard: it brings many small enhancements, and while you may not be immediately struck by the importance of some of the features, this version of OS X adds up to more than the sum of its parts. Combine what Apple has already done with what third party developers will do in their Leopard applications, and this is going to be a very solid platform for the future.

FINDER

The Finder now includes a very clever new trick called Quick Look, which lets you get a glance inside documents without launching their associated application. Select a file in the Finder, hit the space bar, and a window pops up displaying a read-only version of the document. This works for standard files, such as JPegs and PDFs, but developers can add support for their own file formats.

The Quick Look technology also lies beneath the new iTunes-style Cover Flow feature in Finder. Cover Flow presents files as horizontally scrolling images of their contents (just as it does with album art in iTunes). Cover Flow can be useful, but only for certain kinds of files: if you want to shuffle through a folder of images or graphical documents, Cover Flow can certainly help. But if you have a folder of text documents, it's no more than a gimmick. As a programmer, I find it hopeless for finding a particular file of code - the details are just too small to see.

DOCK

This has changed too - though more controversially - from a two-dimensional panel with little black arrows for running applications, to a three-dimensional reflective shelf with little lights. Several notable Mac developers and icon designers complained of earlier betas that the new appearance makes icons designed for the earlier Dock perspective appear incorrect.

Fortunately, in the final version, if you position the Dock on the left or right side of the screen, its appearance changes back to a flat 2D panel almost identical to Tiger's, except it now has a smoked grey appearance instead of the transparent white of earlier systems. (And if you feel brave about typing Terminal code - only one line - you can turn off the "shelf" feel on the bottom too: see this explanation).

SPACES

Leopard adopts a desktop feature which has been available on other operating systems for many years - well, Unix systems; Windows never quite managed it. It's an implementation of 'virtual desktops' which it calls Spaces. This means you have multiple workspaces, each of which can contain particular applications or file windows. You can shift between them using a hotkey (F8 is the default) or a flick of the mouse to a chosen corner.

This is a powerful feature for users who multitask heavily between different applications. You might want to create one space for reading email and web feeds, another for web research, another for word processing and a final one for iTunes. A nice feature of Apple's implementation is that you can zoom out from any workspace for a bird's eye view of all your spaces. This neatly solves one of the common problems with most virtual desktop systems: you forget where you put a particular window or application.

Personally, I'm still getting used to Spaces; I'm more used to the traditional Mac tricks for decluttering the screen, such as hiding unused applications (for Windows users, it's like minimising, without minimising). If you're coming to Mac OS X from an operating system which has virtual desktops, you'll feel right at home.

STACKS

Leopard's Dock also brings a new feature called Stacks. A stack is created by placing a folder in the Dock: when clicked it presents a window showing the files' icons contained in that folder. It's small and subtle, but will significantly increase the usefulness of the Dock as a file and folder shortcut bar. When the stack window appears, you can drag items from that window to other places.

Imagine you're creating an email message and you want to attach a file. Previously, that would involve leaving the Mail program, navigating to your file in the Finder and dragging it into the message window. In Leopard, all you have to do is pop open the Documents stack and drag the file to the message window. Subtle, but a useful time saver.

SAFARI

Apple's browser has received a major upgrade, to version 3.0, whose benefits will also be coming to those who stick with Mac OS X 10.4. Its headline feature is that you can take a fragment of a web page and turn it into a Dashboard widget, which will automatically update as the web page it belongs to does. There's also a more subtle improvement: you can resize text areas in web pages. You've probably had the experience where a web site provides a tiny little text box and asks you to write a thousand words. Safari 3 lets you drag the corner of any text area on any web page and reshuffles everything else to make room.

SPOTLIGHT

Apple's system-wide search technology has broadened its reach. Previously confined to searching one computer, Spotlight can now search any networked computer to which you have access. As I use two Macs for different purposes, it's great to be able to search machines elsewhere in the house from my laptop. The results aren't intermingled; you have to choose whether to search "This Mac" or a connected server, and you don't see both sets of results in the same window.

One drawback of the Spotlight implementation is that you have to connect to another Mac as a known user before you can search it. This is done for obvious security reasons, but does demand some setting up first. It also makes it pretty tedious to attempt a search across a large number of Macs, as you have to connect to each in turn and then search.

In Mac OS X 10.4, Spotlight was often criticised for slow performance. In Leopard, search responsiveness has been dramatically improved. I used to use a third-party search tool called LaunchBar to find programs and files on my Mac, but I've found that Leopard's Spotlight is just as fast for finding and launching applications. Apple has also made the built-in Dictionary searchable, so you can now find definitions of words through Spotlight.

TIME MACHINE

Leopard aims to change all of our poor (admit it) backup habits with a technology called Time Machine, an automatic backup system. We all know we should back up, but doing so can be inconvenient and confusing. If you have an extra hard disk, Time Machine keeps a complete and automatic backup of your system and all your files. If you need to retrieve an older version of a file, Time Machine provides a dramatic animated interface that presents your backup files flying backwards in a star field; backup and restore has never been so pretty.

Time Machine also tracks which files that have changed since your last backup, which is really useful for laptops that might not always be connected to a separate hard disk. When you get home and connect your backup drive, Time Machine doesn't need to compare every file on your laptop to every file in your backup: it already knows which files changed and can update the backup efficiently. It can even backup wirelessly. The ease of use and unobtrusiveness of Time Machine is a subtle, but ultimately dramatic, revolution in backup usability.

MAIL

Apple's Mail application has also had some attention. Catching the recent wave of enthusiasm for personal productivity tools, Apple has built in a to-do list manager and notes. This is more than a simple feature tacked-on - Leopard has a system-wide database of To-Do items, which is also used by iCal and available to third-party developers.

This means that you can enter to-do items in Mail, check them off in iCal and both applications will see changes made in the other. This is pretty useful - although it's not immediately obvious that this part of both iCal and Mail are connected. I find it useful to create To-Do items right in Mail, since so many of those To-Dos are generated by email messages.

It will take some time to realise the full power of this feature, but eventually it will become as important and useful as the shared Address Book database has been in previous versions of Mac OS X. With the system-wide Address Book, your contacts' data can appear in Mail, in iChat and even in networked games. With a system-wide To-Do list, other applications such as project planners and reminder programs can access your To-Do items and use them in those applications. Having To-Do items in Mail doesn't diminish iCal's usefulness; it just gives you another way to create these tasks.

ICAL

Along with Mail, Apple's calendar program iCal has been enhanced. If you work in a Mac-based office, you'll be interested in the support for the new CalDAV standard. If your business uses Mac OS X Server, the new iCal Server in Leopard means iCal users can schedule meetings together and share calendars rather like Microsoft Exchange users. Solo iCal users will appreciate the new inline event editor, which makes entering appointments much more efficient.

ICHAT

Apple's instant message client iChat includes a new screen sharing feature, essentially a limited one-to-one version of the more sophisticated Remote Desktop program that Apple sells to system administrators. It lets you see someone else's desktop - subject to their permission - and control their programs. You'll likely find this easier than working blind trying to help a relative over the phone. The idea of sharing your screen extends to being able to share other content via iChat Theater, which lets you show someone else both the contents of your screen and the video from a webcam over the internet. This is really useful for discussing a presentation or showing a photo slideshow.

CORE ANIMATION

One of the underlying technologies in Leopard is Core Animation. It's not a feature per se, but we Mac developers are very excited about its possibilities. From its earliest days, Mac OS X has been known for its dynamic user interface, but developers have sometimes found it difficult to implement the visual effects. Core Animation relieves us of that burden. It provides animations, fades, dissolves and other visual effects built in to the operating system and thus available without extra work to developers.

Apple's use of Core Animation in Leopard has mostly been confined to "subtle and useful" but, just as some people went crazy with multiple fonts in the 1980s, some developers will surely go overboard with the visuals. In time, developers and the market will understand where animation fits into the overall user experience.

In conclusion, Leopard is going to be a great release. In my opinion, Time Machine is going to be a great stealth hit. It won't change your life on the day you install it, but milllions of users will turn it on, forget it and one day be very grateful that it has been sitting in the background keeping their files safe all the while.

I love the improvements to the Finder and predict that I'll be addicted to Quick Look within an hour of switching to Leopard. Personally, I'm not expecting to get a lot of use out of the iChat enhancements, or the fancy HTML templates in Mail. As a Mac programmer, though, I can tell you that there are a ton of things built in to Leopard for programmers to play with; Core Animation is just the start. You won't notice these improvements today, but in the long run, you'll start to see great things happening on your Leopard-running Mac.

· Fraser Speirs runs a company, Connected Flow, which produces plugins for exporting images from iPhoto and Aperture to Flickr.